Due to the larger bandwidth available for New Radio (NR, e.g., in the mmWave spectrum) compared to LTE, along with the native deployment of massive MIMO (Multiple-Input Multiple-Output) or multi-beam systems in NR, integrated access and backhaul (IAB) links can be developed and deployed. This may, for example, allow easier deployment of a dense network of self-backhauled NR cells in a more integrated manner by building upon many of the control and data channels/procedures defined for providing access to (user equipment's (UEs). In general, an integrated access and backhaul link multiplexes access (mobile terminal/e.g., user equipment) and backhaul (distributed unit/e.g., access point) links in time, frequency, and/or space (e.g., beam-based operation) to relay user traffic to a donor or parent IAB node, and vice-versa.
The design of a multi-hop IAB in 3GPP is based on a hierarchical concept, which allows using the various downlink and uplink procedures and channels to create a multi-hop network. This can be done by having a user equipment (UE) function (IAB-UE) and a gNB or distributed unit (DU) function (IAB-DU) at each relay. The IAB-UE function is used for communicating with the parent node(s), whereas the IAB-DU function is used for communicating with the child nodes or a UE as shown in FIG. 2 (illustrating half-duplex constraint at the relay). The UE function within the relay node is also referred to as MT (mobile terminal).
An advantage of IAB is that backhaul and access are integrated and multiplexed in the scheduler, allowing dynamic resource allocation between the backhaul and access links (in both downlink and uplink directions). As a result, the duplex constraint at the relay that prevents simultaneous transmission and reception is a factor when considering how to multiplex access and backhaul links. This consideration can become more significant when supporting multiple hops of backhaul links, each with a similar duplex constraint. For example, the latency/overhead introduced by orthogonal partitioning of resources in either time or frequency can be considered. In particular, with mmWave frequencies which are typically TDD (Time Division Duplex), a practical scenario for initial IAB deployments is to enforce a half-duplex constraint at the relay, wherein the nodes transmit on the access link and/or backhaul link at any given time.
Time-division-multiplexing (TDM)/frequency division-multiplexing (FDM) may be used with access and backhaul links. TDM partitioning has downlink/uplink switching gaps between both the backhaul directions as well as for the access links, while a guard band is introduced between backhaul subframes in the case of FDM.
Furthermore, the native deployment of massive MIMO systems in NR also allows supports a complementary multiplexing technique of spatial reuse (e.g., spatial division multiplexing, or SDM) between the backhaul and access links. For example, while still assuming a half-duplex constraint at the eNB/relay, it is possible that uplink access traffic can be received while simultaneously receiving backhaul traffic. Likewise, the downlink access traffic can be served while also transmitting backhaul/relay traffic. Depending on the backhaul frame structure and support for beamforming, the access and backhaul traffic can be transmitted using orthogonal resources or by multi-user MIMO transmission schemes.